This invention relates to fuel cell systems and, in particular, to fuel cell systems having high efficiency.
There have been attempts recently by designers of fuel cell systems to develop high efficiency systems by combining the fuel cell with a turbine generator. These so-called hybrid systems typically situate the fuel cell in the position normally occupied by the combustor of the turbine generator. Air is compressed by the turbine generator compressor section, heated and then sent to the fuel cell cathode section. The anode section of the fuel cell, in turn, is supplied with pressurized fuel.
With this configuration, both the fuel cell and the turbine generator produce electricity. More particularly, the fuel cell operating at pressure electrochemically converts the pressurized fuel and oxidant gases to produce electricity and pressurized flue gas. The latter gas is then expanded in the turbine generator expansion section to produce more electricity.
The above hybrid system while providing efficiencies also has a number of disadvantages. One disadvantage is that the fuel cell is operated at pressure. For high temperature fuel cells this substantially increases the cost of the power plant hardware. It also inhibits the use of internal reforming in the fuel cell. This further increases the plant cost and reduces efficiency. Finally, it subjects the fuel cell to potentially damaging pressure differentials in the event of plant upset.
Another disadvantage is that the turbine generator must operate within the pressure limits of the fuel cell. This pressure range may not result in the most efficient pressure ratio for the turbine. A further disadvantage is that the fuel cell cannot be operated without the turbine generator and the turbine generator cannot be operated without the fuel cell. This total dependence of the fuel cell and the turbine generator on each other decreases the reliability of the system.
Another hybrid system has been proposed in which the fuel is supplied at ambient temperature allowing the use of an internal reforming or direct fuel cell. In this system waste heat from the fuel is transferred to a fired turbine generator (Brayton) cycle.
More particularly, anode exhaust from the anode section of the fuel cell and a portion (50 percent) of the low pressure oxidant gas developed by the expansion section of the turbine generator are burned in a burner. The output oxidant gas from the burner is at a high temperature (2,000 degrees F.) and is passed through a heat exchanger adapted to operate at the high temperature. The cooled gas at a lower intermediate temperature (1,250 degrees F.) from the heat exchanger is then carried by a blower and combined with the other portion (50 percent) of the oxidant gas developed by the expansion section of the turbine generator. The combined gas at a low pressure (15 psia) is then input to the to cathode section of the fuel cell.
In the aforesaid system, the compressor section of the turbine compresses air to a high pressure (360 psia) and the resultant pressurized gas is then fed to the high temperature heat exchanger and heated to a high temperature. The heated and compressed air stream is burned in another burner with a small portion of the fuel gas (5 percent) and the burner output (at 2,000 degrees F.) is sent through the expansion section of the gas turbine. This results in cooler (720 degrees F.), decompressed, but still pressurized, output oxidant gas which is used, as above-described, to develop the oxidant gas for the fuel cell cathode section.
Also, in the above system, the exhaust from the cathode section of the fuel cell is fed to a heat recovery steam generator. The latter generator recovers heat from the gas stream to develop steam which is fed to a steam turbine.
As can be appreciated, the aforesaid system requires the use a very high temperature heat exchanger which has yet to be fully developed. This coupled with the complexity of the system, i.e., its use of blowers, recycle streams and two turbines, reduces the attractiveness of the system as a commercially viable hybrid system.
It is therefore an object of the present invention to provide a fuel cell system of the above-mentioned hybrid type which avoids the disadvantages of the prior systems.
It is a further object of the present invention to provide a fuel cell system of the hybrid type which is less complex and more efficient than prior systems.
It is yet a further object of the present invention to provide a fuel cell system of the hybrid type which realizes the aforementioned objectives and permits the use of a high temperature fuel cell.
In accordance with the principles of the present invention the above and other objectives are realized in a fuel cell system in which a high temperature fuel cell having anode and cathode sections and a heat engine having compressor and expansion cycles are employed. More particularly, in accord with the invention a heat recovery unit receives fuel and water from respective fuel and water supplies, and also cathode exhaust gases from the cathode section of the fuel cell. The output stream from the heat recovery unit contains heated fuel and serves as the fuel supply for the anode section of the fuel cell.
The heat recovery unit also acts as a heat exchanger for the pressurized oxidant gas supplied from the compressor cycle of the heat engine which receives air. The heated pressurized oxidant gas is passed through a heat exchanger where it is further heated before being acted on by the expansion cycle of the heat engine. In this cycle, the gas is expanded to a low pressure and fed to an oxidizer which also receives the exhaust from the anode section of the fuel cell.
The output gas of the oxidizer is passed through the heat exchanger and gives up heat to the pressurized gas passing therethrough from the compressor cycle of the heat engine. The cooled gas is then fed into the cathode section of the fuel cell as the cathode supply gas.
With this configuration of the hybrid system, the complexity of the system is significantly reduced. At the same time, heat exchange operations can be performed at temperatures for which conventional equipment is available. Also, pressurization of the fuel gas is not needed and oxidant gas is supplied at pressurization resulting form the heat engine only. These and other advantages of the system will be discussed in more detail hereinbelow.